flag package in Go - do I have to always set default value?

Question

Is it possible not to set default value in flag package in Go? For example, in flag package you can write out the following line: ``` filename := flag.String("file", "test.csv", "Filename to cope with") ``` In the above code, I don't want to necessarily set default value, which is `test.csv` in this case, and instead always make users specify their own filename, and if it's not specified then I want to cause an error and exit the program. One of the way I came up with is that I first check the value of `filename` after doing `flag.Parse()`, and if that value is `test.csv` then I have the program exits with the appropriate error message. However, I don't want to write such redundant code if it can be evaded - and even if it can't, I'd like to hear any better way to cope with the issue here. You can do those kind of operations in Python's `argparse` module by the way - I just want to implement the similar thing if I can... Also, can I implement both short and long arguments (in other wo…

Accepted Answer

I think it's idiomatic to design your flag values in such a way which implies "not present" when equal to the zero value of their respective types. For example:

```
optFile := flag.String("file", "", "Source file")
flag.Parse()
fn := *optFile
if fn == "" {
        fn = "/dev/stdin"
}
f, err := os.Open(fn)
...
```

Ad the 2nd question: IINM, the flag package by design doesn't distinguish between `-flag` and `--flag`. IOW, you can have both `-f` and `--file` in your flag set and write any version of `-` or `--` before both `f` and `file`. However, considering another defined flag `-g`, the flag package will not recognize `-gf foo` as being equvalent of `-g -f foo`.

flag package in Go - do I have to always set default value?

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